Thermoplastic resin films such as polyamide films and polyester films have excellent strength, transparency, and moldability, and are consequently widely used as packaging materials. However, because these thermoplastic resin films also exhibit reasonably high levels of permeability to gases such as oxygen, if this type of thermoplastic resin film is used for packaging general foodstuffs, retort foods, cosmetics, pharmaceuticals, or agricultural chemicals or the like, then during long-term storage, gases such as oxygen can permeate through the film, causing deterioration of the package contents.
As a result, laminated films produced by coating the surface of a thermoplastic resin with an emulsion or the like of polyvinylidene chloride (hereafter abbreviated as PVDC), thereby forming a PVDC layer with good gas barrier properties, are widely used for applications such as food packaging. However, PVDC generates substances such as acidic gases on incineration, and with recent advances in environmental awareness, there is considerable demand for replacing PVDC with other materials.
One example of an alternative material to PVDC is polyvinyl alcohol (hereafter abbreviated as PVA), which does not generate toxic gas, and exhibits excellent gas barrier properties under low humidity conditions. However, as the humidity increases, the gas barrier property declines rapidly, so that in most cases, PVA films cannot be used for wrapping foods that contain moisture.
One example of a polymer known to improve upon the deterioration in gas barrier properties seen for PVA under high humidity conditions is a copolymer of vinyl alcohol and ethylene (hereafter abbreviated as EVOH). However, in order to ensure that the gas barrier property is maintained at a practical level under high humidity, the proportion of ethylene within the copolymer must be increased beyond a certain level, but unfortunately, the resulting polymer becomes difficult to dissolve in water. Accordingly, in order to produce a coating agent using EVOH with a high ethylene ratio within the copolymer, either an organic solvent, or a mixed solvent of water and an organic solvent must be used, but this is undesirable from an environmental viewpoint, and also results in increased costs due to the necessity of providing a process for recovering the organic solvent.
Many techniques have been investigated with the aim of developing a liquid composition comprising a water-soluble polymer that can be coated onto a film to form a coating that exhibits favorable gas barrier properties even under conditions of high humidity. Reference 1 (Japanese Laid-Open Publication No. Hei 06-220221), reference 2 (Japanese Laid-Open Publication No. Hei 07-102083), reference 3 (Japanese Laid-Open Publication No. Hei 07-205379), reference 4 (Japanese Laid-Open Publication No. Hei 07-266441), reference 5 (Japanese Laid-Open Publication No. Hei 08-041218), reference 6 (Japanese Laid-Open Publication No. Hei 10-237180), and reference 7 (Japanese Laid-Open Publication No. 2000-000931) all disclose techniques using mixtures of PVA and either polyacrylic acid or polymethacrylic acid.
However, in the inventions proposed in the above references 1 through 7, either heat treatment at high temperature, or heat treatment over an extended period, is required to produce the desired favorable gas barrier property, meaning a large amount of energy is required during production, which places a significant burden on the environment. Furthermore, if a high temperature heat treatment is employed, then not only is there an increased danger of color changes or decomposition of the PVA and the like which constitute the barrier layer, but wrinkling can also occur in the plastic film substrate or the like onto which the barrier layer is laminated, and deformation such as curling or shrinkage can also occur, making the product unsuitable as a packaging material. In order to prevent deterioration of the plastic substrate, a special heat resistant film that is capable of withstanding the high temperature heat treatment must be used as the substrate, but this creates problems of practicality and economic viability. On the other hand, if the temperature of the heat treatment is low, then treatment must be conducted over extremely long periods, causing an undesirable fall in productivity.
Furthermore, investigations have also been conducted into resolving the above problems associated with PVA film by introducing cross-linking structures into the PVA. However, although the humidity dependence of the oxygen gas barrier property of PVA film typically decreases with increasing cross-linking density, the inherent oxygen gas barrier property of the PVA film under dry conditions tends to deteriorate, meaning it is extremely difficult to achieve a favorable oxygen gas barrier property under high humidity conditions.
Cross-linking of polymer molecules generally improves the water resistance, but the gas barrier property describes the ability of the material to prevent the penetration or diffusion of comparatively small molecules such as oxygen, and a favorable gas barrier property can not always be achieved simply by cross-linking the polymer. For example, three dimensional cross-linked polymers such as epoxy resins and phenol resins do not exhibit effective gas barrier properties.
In addition, inventions have also been proposed for producing gas barrier laminates using PVA and a maleic acid-based copolymer, which allow heat treatment to be conducted at a lower temperature, or for a shorter period, than conventional processes (reference 8: Japanese Laid-Open Publication No. 2001-323204, reference 9: Japanese Laid-Open Publication No. 2002-020677, and reference 10: Japanese Laid-Open Publication No. 2002-241671).
The inventions disclosed in these references 8 through 10 enable the production of gas barrier laminates under milder conditions than the inventions disclosed in the references 1 through 7. However, further improvement is still needed in the level of gas barrier properties achieved.